Many wells have been drilled through the "Irpine" unit formations in the Val D'Agri field development since the start of the project. The Irpine unit contains overpressured shales and reactive claystones. In the early years, simple non-inhibitive lignosulfonate fluids were used leading to large washouts. Later, potassium-based fluids, including potassium chloride and potassium carbonate-based systems, were used. None of these fluids provided adequate borehole stability, despite applying high mud weights. Despite the operator going through many optimisation cycles to improve the performance of these systems, hole problems persisted.

Formation stability issues while drilling the 12 ¼" hole for 9 • " casing on the Monte Enoc 10 Or B well prompted the operator and the drilling fluids supplier to search again for an alternative fluid system. This led to the development and deployment of a chloride- and potassium-free, high-performance water-based mud (HPWBM). This was the first time a potassium-free fluid was used in this field.

The HPWBM delivered better improved hole stability and increased rates of penetration, as well as reduced dilution rates. The operator was able to complete the section well within budget with the 9 • " casing placed as programmed without issues.

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